The present invention relates generally to a photo detector unit, and, more particularly, to a photo detector unit that detects light in a so-called immersion exposure apparatus, which fills a liquid or fluid in a space between a final surface of a projection optical system and a surface of a plate to be exposed, and exposes the plate via the projection optical system and the liquid using light from a light source.
A conventional projection exposure apparatus projects a circuit pattern of a reticle (mask) onto a wafer or another plate via a projection optical system. A high-resolution and high-quality exposure apparatus has recently been increasingly demanded.
The immersion lithography is one attractive means that satisfies the high-resolution demand. The immersion lithography promotes an increased numerical aperture (“NA”) by replacing a medium (typically, air) at the wafer side of the projection optical system with the liquid. The NA of the projection optical system is defined as NA=n·sin θ, where n is a refractive index of the medium. The NA increases up to n when the medium has a refractive index higher than the refractive index of air, i.e., n>1. Thus, the immersion lithography can reduce a resolution R of the exposure apparatus, which is defined as R=k1 (λ/NA), where k1 is a process constant, and λ is a wavelength of a light source.
On the other hand, high-quality exposure requires measurements of the exposure dose, light intensity distribution, and the physical quantity of the optical performance of the projection optical system, such as a wavefront aberration, and necessary adjustments based on the measurement result. In addition, the immersion exposure apparatus requires measurements through the liquid. In this case, a photo detector, which is typically made of a semiconductor device, is vulnerable to the humidity and, thus, should be isolated from the liquid. One conceivable protection of isolating the photo detector from the liquid is to use a light transmitting window. Since the photo detector cannot bring its photosensitive surface into contact with the window due to its uneven surface and mounted wiring, such as wires, an air gap is inevitable between the window and the photosensitive surface. The photo detector used for the immersion exposure apparatus needs to receive the light having an NA greater than one, which cannot reach the photo detector due to the air gap.
The prior art includes Japanese Patent Application, Publication No. 2005-79587, which proposes a sensor that includes a photo-diode (photocell) and a fluorescent substance (luminescence layer), as shown in FIG. 16. This reference teaches to insert a light transmitting filler sheet, and to introduce the light to the fluorescent substance without loss. Other conventional photo detector units are proposed in Japanese Patent Applications, Publication Nos. 2003-031467 and 2004-251764.
However, Japanese Patent Application, Publication No. 2005-79587, merely expects a damped-wave tunneling effect, and does not provide any specific solution to the light having an NA of one or greater. This reference cannot measure all the doses or provide a highly precise measurement. Japanese Patent Applications, Publication Nos. 2003-031467 and 2004-251764, do not contemplate an immersion exposure apparatus, and cause the characteristics of the photo detector to deteriorate, because the photo detector contacts the liquid when the photo detector is applied to the immersion exposure apparatus.